The above-noted commonly owned and co-pending U.S. Pat. Nos. 4,624,798 and 4,624,865 disclose novel electrically conductive microparticles in the form of metal coated magnetic ceramic microspheres, methods for producing these microspheres, and also describe their usefulness in forming electrically conductive coatings or composites for various applications. We have now discovered novel infrared absorbing compositions consisting of microparticles coated with certain inorganic materials.
Various materials, such as carbon black and other dark pigments, have been used in the past to absorb infrared radiation. While various pigments used in the past absorb a portion at the infrared region, they become highly reflective in other portions of the infrared. These compositions all suffer the disadvantage that the binder used to hold the pigment is reflective in the infrared region and detracts from the effectiveness of the absorber particle. These systems utilize a dispersion of the absorber particles such that the energy must penetrate some distance into the binder to reach the particles. A further disadvantage of this approach is the difficulty of simultaneously obtaining the critical dispersion distance for the particles and the strict requirements for the particle size. Prior technology utilizing such pigments is unable to obtain uniform and strong absorption over the 0.5 to 25 micron bandwidth. Coatings utilizing the prior art could not be integrated into ultrawide band electromagnetic absorbers due to the inherent high dielectric constant of the pigments which cause high reflectivity of the coatings in the microwave region of the electromagnetic spectrum.
The present invention provides a strong, light-weight composition containing coated microparticles. The composition has very broad band absorption properties for electromagnetic radiation in the infrared region and is effective when used as a thin coating or as a structural composite material. It can withstand very high temperatures (900 degrees C.) and is generally limited only by the binder in this respect. In contrast to the prior art, these coated particles require much less of the reflective binder to achieve good physical properties and the size of the coated particles is such that the particles extend above the binder surface exposing the absorber to the incident radiation with little interference by the binder. The composition has the advantage of being transparent in the microwave region and may readily be used in conjunction with other absorbers to obtain an absorber which is effective in both the microwave and infrared regions. It has the added advantage of absorbing sound and is a good thermal insulator displaying decreased emissions when placed over substrates radiating in the far infrared. It may be easily applied to a substrate with conventional spray equipment.